1. Field of the Invention.
This invention relates to an improved suspension for heavily loaded haul trucks.
2. Setting of the Invention.
As is well known, it has been difficult to provide proper suspension for heavily loaded off highway haul trucks due to the great disparity between the wheel loads in the loaded and unloaded condition. Suspension systems that enhance the driver comfort when the truck is unloaded tend to be hard on the truck frame and those type suspension systems that offer better characteristics in the loaded condition as far as the frame and the other structure is concerned, tend to be harsh and rough on the driver in the unloaded state.
Over the past twenty (20) years or so, off highway trucks have generally used one of three types of suspensions. These are:
(1) Rubber columns; PA1 (2) Gas and oil systems; PA1 (3) Silicon fluids.
The initial rubber spring suspension was based on rubber pads of the type used in railroad drawbar shock absorbers. A stack of these pads generates a load deflection curve very nearly ideal for the application. That is, at low loads the spring rate is low making the natural frequency low which is desirable for driver comfort. The problem is that the heat generated from its constant compression and expansion ages the rubber very rapidly and it loses the resiliency required for a low spring rate. This results in a high natural frequency when a vehicle is unloaded and thus, a rough ride for the driver. Another type rubber spring uses a belted rubber cylinder to produce a similar load/deflection curve but it also suffers from aging and loss of resiliency.
The gas/oil type suspension system makes use of the compressibility of an inert gas to provide the spring rate and the oil to provide a damping fluid. Since the spring rate of this type suspension system varies inversely to the pressure squared, the frequency curve approaches the ideal i.e., load. However, the load deflection curve for a reasonable volume of gas has too much curvature resulting in stiff spring on the heavily loaded part of the curve with very little allowance for high overloads.
The silicon fluid spring uses the compressibility of silicon as a spring element. This has the advantage of also using the fluid for a dampening fluid. The compressibility of this fluid is not high so large volumns are required if reasonable strokes are used. Also the compressibility of the fluid is very near linear so some compromise must be made on the spring frequency rate, usually resulting in a rough ride when the truck is empty.